From 13:10 to 13:50 of the video, I can see that the guy compiled the lisp code in one of the windows using C-c C-c, and its immediately available in the other window. What setting do I have to change so my emacs+slime behave in the same way?

Currently, I cut and paste code from between windows, and that is really tiresome. I can only run the code function in the repl if I load the file in the repl, otherwise, If I compile the code in a different window (which does compile without errors), it is undefined in the repl window.

I'm using CCL by the way, and using a PC with Windows 10 OS. Thanks in advance.